1. Field of the Invention
The present invention relates to a fluorescent lamp driving method and apparatus for driving, for example, a fluorescent lamp by stabilizing a current flowing in the fluorescent lamp for a change in input voltage.
2. Description of the Related Art
FIG. 4 is a circuit diagram showing a fluorescent lamp driving circuit of the related art which includes an inverter circuit 51 for driving cold cathode fluorescent lamps (CCFLs) 52.
In a backlight for a large liquid-crystal-display panel, the CCFLs 52 are used. In order for the CCFLs 52 to emit light, the inverter circuit 51 generates a high-voltage alternating current having several tens of kHz.
The CCFLs 52 have negative resistance characteristics. In order to drive the CCFLs 52 in parallel using a single inverter circuit (transformer), a function of balancing a current flowing in each CCFL is necessary.
Accordingly, in the fluorescent lamp driving circuit shown in FIG. 4, ballast capacitors Zc are used for the function of balancing the current flowing in each CCFL in the case of driving the CCFLs 52 in parallel.
A backlight unit provides a luminance necessary for an apparatus and needs to maintain the luminance stable. The inverter circuit 51 has a control function of maintaining a current sum Io of currents necessary for the CCFLs 52 to be constant.
The impedance of the CCFLs 52 is changed by a temperature or a current. Thus, by performing pulse-width-modulation (PWM) control (changing a ratio between a conduction time and a non-conduction time) that changes conduction times of switching elements Q1 and Q2 of the inverter circuit 51, or pulse-frequency-modulation (PFM) control that changes a driving frequency, an output voltage Eo of an inverter transformer T1 is changed, whereby the sum Io of the currents necessary for the CCFLs 52 is controlled to be constant.
FIG. 5 is a characteristic chart showing a relationship between the ambient temperature of the CCFLs 52 of the related art and the terminal voltage across ends of the CCFLs 52.
As shown in FIG. 5, the impedance of the CCFLs 52 is changed by an operating temperature. When the temperature of the CCFLs 52 increases, the impedance decreases. This change in impedance of the CCFLs 52 appears also as a change in phase angle of the impedance of the CCFLs 52. The sum Io of the currents necessary for the CCFLs 52 is the sum (represented by I1+I2+I3 . . . ) of currents I1, I2, I3, . . . in the CCFLs 52. In other words, the sum current Io is represented by the sum of current vectors I1, I2, I3, . . . in accordance with the absolute values and phase angles of the CCFLs 52.
As a result, even if the magnitude of a current Iin supplied from the inverter circuit 51 is controlled to be constant by changing an output voltage Eo of the inverter transformer T1, a phenomenon occurs in which the current vectors I1, I2, I3, . . . flowing in the CCFLs 52 vary with a changes in the impedance of the CCFLs 52.
FIG. 6 is a characteristic chart showing a state in which a high voltage applied to one CCFL 52 flows out as a leak current -Ile through a distributed capacity existing between a close conductor, such as a chassis and reflector included in the backlight unit, and the CCFL 52. The chart also shows the amount of the leak current -Ile.
As shown in FIG. 6, a high-voltage-applied side has a larger amount of the leak current -Ile. The leak current -Ile causes a capacitive load, so that the phase is ahead. If a load characteristic of the CCFL 52 is only a resistive component, a current Iout flowing only in the CCFL 52 is in phase with the voltage. The current Iin supplied from the inverter circuit 51 to the CCFL 52 is a resultant current Iout+Ile.
FIG. 7 is a vector diagram showing the current Iin supplied from the inverter circuit 51 to the CCFL 52.
As shown in FIG. 7, the current Iin supplied from the inverter circuit 51 to the CCFL 52 is a resultant current of the current Iout flowing only in the CCFL 52 and the leak current -Ile. The load of the CCFL 52 is not a pure resistance and the load has a large capacitive component. Thus, the phase of the resultant current Iin has to be ahead of the voltage, compared with the case of FIG. 7.
When the operating temperature of the CCFL 52 increases, the impedance decreases. If this change is resistive, a resistance value is small. Thus, the phase of the current Iin supplied from the inverter circuit 51 to the CCFL 52 lags behind.
The inverter circuit 51 controls the current Iin (the sum current Io) supplied from the inverter circuit 51 to the CCFL 52 to be constant, the current Iin being the sum current Iout+Ile. Thus, when the phase φ of the impedance of the CCFL 52 changes, as indicated by the dashed line in FIG. 7, the phase φ of the current Iin changes from φ1 to φ2. The current Iout flowing only in the CCFL 52 changes from Iout* to Iout**. In addition, the leak current -Ile changes from -Ile** to -Ile*.
As described above, even if the magnitude of the current Iin (the sum current Io) supplied from the inverter circuit 51 to the CCFL 52 is controlled to be constant, the impedance of the CCFL 52 changes due to ambient temperature and self-heating.
The amount of this change changes depending on the type and characteristics of a fluorescent lamp. In addition, a change ΔIout of the current Iout flowing only in the fluorescent lamp is greatly influenced by a phase difference between the voltage and current output from the inverter circuit 51.
As an inverter for driving a fluorescent lamp by stabilizing a current flowing in the fluorescent lamp, there is an inverter (see Japanese Unexamined Patent Application Publication No. 2004-335362) which stabilizes a current flowing in a fluorescent lamp by detecting and controlling the current, and in which, when ambient temperature of the inverter exceeds a set temperature, the current is decreased as the ambient temperature increases.